Mesoscale Precipitation Discussion 0686
NWS Weather Prediction Center College Park MD
1051 PM EDT Fri Aug 17 2018

Areas affected...Central/Eastern Oklahoma, Western Arkansas

Concerning...Heavy rainfall...Flash flooding possible

Valid 180230Z - 180745Z

Summary...Thunderstorms should continue to develop near the
Oklahoma-Arkansas border and may begin to slowly sink southeast
overnight. These storms could produce locally heavy rainfall in
excess of 3 inches, which could lead to flash flooding. Ground
conditions would favor flash flooding closer to I-40 and Fort
Smith, where heavy rainfall in the past week has been focused.
Further west, into central Oklahoma, scattered thunderstorms may
persist for another few hours. Although these storms may be less
organized, they could move slowly enough to lead to very localized
areas of heavy rainfall. The risk of flash flooding would be lower
in those areas, but should still exist for another couple hours.

Discussion...KSRX radar shows a developing convective line
oriented west-east just south of Interstate-40 near the OK-AR
border. This line was developing in a region of confluent
925-850mb flow that should persist overnight, and provide a focus
for renewed convective growth. The line was also situated on the
eastern periphery of a reservoir of very strong instability, and
the low-level flow just above the surface has a substantial
westerly component as it begins to approach the confluence zone.
Therefore, thunderstorms should have a tendency to backbuild, and
most convection-allowing models show this process in some form
continuing overnight in far eastern Oklahoma and into Arkansas. As
the convection becomes better established, it should gradually
sink south as it trains, consistent with a northerly component to
the deep layer mean wind and Corfidi vectors. The flash flood risk
appears to be highest in association with this activity, given the
favorable mesoscale setup for training. Precipitable water values
around 1.7 inches (per CIRA blended TPW) should support rain rates
around 1-2 in/hr, and if sustained for a couple hours these could
exceed flash flood guidance.

Further west, scattered convection was developing near the
instability max, where RAP analysis showed MUCAPE values over 4000
j/kg. The limiting factor in central Oklahoma would be a lack of
strong forcing to focus convective development in a particular
area. Thus far, thunderstorms that have developed have not been
sustained for a particularly long period of time. Nevertheless,
given similar PWs, the rain rates could reach 1-2 in/hr and
produce localized flooding. The risk of flash flooding is lower,
but cannot be ruled out, particularly with merging or splitting
cells that could have motions that deviate from the mean flow. One
feature of interest to be watched is a northward propagating
outflow boundary pushing toward OKC metro area on KTLX radar. This
boundary is beginning to reach some stronger approx 10 knot
northeasterly flow over the metro area, which could begin to slow
its progress and increase convergence. If the boundary was to
begin to stall and convergence could be sustained in a particular
area for even a couple hours, convective growth could be
sufficiently concentrated to lead to a flash flood risk. However,
convective coverage along the boundary thus far has been somewhat
limited, so that outcome is far from certain.

Lamers

ATTN...WFO...LZK...OUN...SHV...TSA...

ATTN...RFC...ABRFC...LMRFC...

LAT...LON   35579593 35569443 35039256 34009236 34029385
            34369503 34259749 34979820 35549768